Ultraviolet photoproducts at the ochre suppressor mutation site in the glnU gene of Escherichia coli: relevance to "mutation frequency decline" |
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Authors: | Nancy Garvey Evelyn M. Witkin Douglas E. Brash |
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Affiliation: | (1) Waksman Institute of Microbiology, Rutgers University, 08855 Piscataway, NJ, USA;(2) Laboratory of Human Carcinogenesis, National Cancer Institute, 20892 Bethesda, MD, USA;(3) Present address: Department of Chemical Biology and Pharmacognosy, Rutgers University, 08855 Piscataway, NJ, USA;(4) Present address: Department of Therapeutic Radiology, Yale University School of Medicine, 06510 New Haven, CT, USA |
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Abstract: | Summary Ochre suppressor mutations induced by UV in the Escherichia coli glnU tRNA gene are CG to TA transitions at the first letter of the anticodon-encoding triplet, CAA. Premutational UV photoproducts at this site have long been known to exhibit an excision repair anomaly (mutation frequency decline or MFD), whereby post-irradiation inhibition of protein synthesis enhances their excision and reduces suppressor mutation yields ten-fold. We sought to clarify the basis of this unique repair response by determining the spectrum of UV photoproducts on both strands of a 36 by region of glnU which includes the anticodon-encoding triplet. We found that four different photolesions are produced within the 3 by sequence corresponding to the tRNA anticodon: (i) on the transcribed strand, TC (6–4) photoproducts and TC cyclobutane dimers are formed in equal numbers at the site of the C to T transition, indicating that this site is a hotspot for the usually less frequent (6–4) photoproduct; (ii) on the nontranscribed strand, TT dimers are found opposite the second and third letters of the anticodon-encoding triplet, adjacent to the mutation site; and (iii) on the nontranscribed strand, an alkali-sensitive lesion other than a (6–4) photoproduct is formed, apparently at the G in the mutation site. We suggest that mutation frequency decline may reflect excision repair activity at closely spaced UV lesions on opposite strands, resulting in double-strand breaks and the death of potential mutants. |
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Keywords: | Mutagenesis Cyclobutane dimers (6– 4) photoproducts Purine lesion Lethal excision |
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